Identifying the best material for a given application—catalysts, light-harvesting structures, biodiagnostic labels, pharmaceuticals, and electronic devices—is traditionally a slow and daunting task. The options are nearly infinite, particularly at the nanoscale where material properties—optical, structural, electrical, mechanical, and chemical—can significantly change, even at a fixed composition.

Researchers at Northwestern University and the Air Force Research Laboratory (AFRL) report a novel tool utilizing a combinatorial library, or megalibrary, of nanoparticles in a very controlled way. A combinatorial library is a collection of systematically varied structures encoded at specific sites on a surface.

The tool, reported in a paper in Proceedings of the National Academy of Sciences (PNAS), could potentially rapidly test millions—even billions—of nanoparticles to determine the best for a specific use.

When utilizing traditional methods to identify new materials, we have barely scratched the surface of what is possible. This research provides proof-of-concept that this powerful approach to discovery science works.

—Chad A. Mirkin, the study’s corresponding author

The libraries are created using Mirkin’s Polymer Pen Lithography (PPL) technique which relies on arrays, or sets of data element, with hundreds of thousands of pyramidal tips to deposit individual polymer “dots” of various sizes and composition, each loaded with different metal salts of interest, onto a surface. Once heated, these dots are reduced to metal atoms forming a single nanoparticle at fixed composition and size.

By going small, we create two advantages in high throughput materials discovery. First, we can pack millions of features into square-centimeter areas, creating a path for making the largest and most complex libraries, to date. Second, by working at the sub-100 nanometer-length scale, size can become a library parameter, and much of the action, for example, in the field of catalysis, is on this length scale.

—Chad Mirkin

Mirkin is the George B. Rathmann Professor of Chemistry, Materials Science and Engineering and director of Northwestern’s International Institute for Nanotechnology (IIN).

The new study is a partnership between Northwestern’s IIN and the Air Force Research Laboratory as part of the US Air Force Center of Excellence for Advanced Bioprogrammable Nanomaterials at Northwestern. The team utilized a megalibrary and an in situ Raman spectroscopy-based screening technique called ARES to identify Au₃Cu, a gold-copper composition, as a new catalyst for synthesizing single-walled carbon nanotubes.

Carbon nanotubes are light, flexible and stronger-than-steel molecules used for energy storage, drug delivery, and property-enhancing additives for many plastic materials. The screening process took less than one week to complete and is thousands of times faster than conventional screening methods.

ARES was developed by Benji Maruyama, leader, Flexible Materials and Processes Research Team, Materials & Manufacturing Directorate, Air Force Research Laboratory, along with Rao.

We were able to rapidly zero in on an optimal composition that produced the highest nanotube yield much faster than using conventional methods. The findings suggest we may have the ultimate discovery tool—a potential game changer in materials discovery.

—Benji Maruyama

The research was funded by the US Department of Defense through the Air Force Office of Scientific Research and the Air Force Research Laboratory Materials and Manufacturing Directorate.

Resources

• Edward J. Kluender, James L. Hedrick, Keith A. Brown, Rahul Rao, Brian Meckes, Jingshan S. Du, Liane M. Moreau, Benji Maruyama, Chad A. Mirkin (2018) “Catalyst discovery through megalibraries of nanomaterials” Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1815358116